1. Field of the Invention
The present invention relates to an image radiographing (hereinafter, radiographing including not only X-ray radiographing but also photographing for any wavelength range of radiation) apparatus and system, which acquires the corrected output by conducting arithmetic operation processing of a radiographed output and an offset output from an area sensor.
2. Description of the Related Art
Conventionally, an image photographing apparatus or radiographing apparatus employing a sensor array in which photoelectric conversion elements or TFTs formed of amorphous silicon or poly-silicon on a glass substrate are two-dimensionally arranged has been well known. In these apparatuses, it is common that electric charges photoelectrically transferred by photoelectric conversion elements are driven in matrix with TFTs and transferred to a reading device to read them.
An offset correction and driving method for the image radiographing apparatus with the area sensor array was disclosed in Japanese Patent Application Laid-Open No. 2003-244557. FIG. 15 is a diagram showing the system configuration of an X-ray radiographing system to which the conventional image radiographing apparatus is applied. FIG. 16 is a timing chart showing the operation of the X-ray radiographing system as shown in FIG. 15.
As shown in FIG. 15, the X-ray radiographing system comprises an X-ray generator 303, a detection unit 302 for detecting an X-ray passed through a subject 316, a correction unit 308 for performing a correction processing for the data outputted from the detection unit 302, and an output unit 315 such as a monitor for outputting data processed by the correction unit 308. Moreover, it comprises a control unit 301 for controlling the detection unit 302, the X-ray generator 303 and the correction unit 308, a radiographing condition memory 307 accessible by the control unit 301, a radiographing button 305 for issuing a radiographing command to the control unit 301, and a photo timer 304 having an AE (Auto Exposure) function. The detection unit 302 is provided with an area sensor and a reading device (not shown), and the correction unit 308 is provided with an image memory 309 for storing a radiographed output from the detection unit 302, a correction memory 311 for storing an offset output from the detection unit 302 and an arithmetic operation unit 314.
In the conventional X-ray radiographing system constituted in the above way, the area sensor periodically performs a pseudo reading operation (hereinafter referred to as “dummy reading”) indicated by “I” in FIG. 16 to reduce the dark current, as shown in FIG. 16. And if the radiographing button 305 is pressed, a radiographing request signal is issued from the radiographing button 305 to the control unit 301. The control unit 301, upon detecting the radiographing request signal, enables the detection unit 301 with the area sensor to perform at least one dummy reading operation. Thereafter, the control unit 301 controls the X-ray generator 303 to start X-ray irradiation. The photo timer 304 having the AE function generates an AE signal (pulse) to the control unit 301 at an appropriate timing during X-ray irradiation. The control unit 301 controls the X-ray generator 303 to stop X-ray irradiation, upon detecting the AE signal (pulse), and stores the charge accumulation time T1 including X-ray irradiation time at this time in the radiographing condition memory 307. Then, the control unit 301 controls the reading device of the detection unit 302 to read the image data from the area sensor. And the output at this time is stored as “radiographed output X” in the image memory 309 within the correction unit 308.
Subsequently, the control unit 301 enables the detection unit 302 to acquire an offset output for correction. That is, the detection unit 302 conducts detection for the charge accumulation time T2 in a state where the X-ray is not irradiated, to read the image data and acquire the offset output F under the radiographing conditions stored in the radiographing condition memory 307. The offset output F is stored in the correction memory 311 within the correction unit 308. At this time, the time T2 is coincident with the X-ray irradiation time T1 stored in the radiographing condition memory 307.
Thereafter, the arithmetic operation unit 314 performs an arithmetic operation processing of “radiographed output X” stored in the image memory 309 and “offset output F” stored in the correction memory 311, and outputs the offset corrected image data to the output unit 315 such as monitor.
Generally, the area sensor made of amorphous silicon used for the conventional image radiographing apparatus is less negligibly affected by the dark current from the photoelectric conversion elements. Accordingly, this method has the great effect in which the X-ray irradiation time T1 is stored in the radiographing condition memory and after the radiographed output is acquired, the offset output is acquired under the condition where the accumulation time T2 is coincident with the X-ray irradiation time T1 as in the above example.